Sonic techniques have been extensively employed for geophysical measurements during the past 50 years. The best known of these methods is the technique of generating an acoustic impulse with an explosive source near the surface of the earth and then detecting and recording the resulting reflected acoustic signals which characterize subterranean layers.
A later development of this basic approach includes the use of a varying-frequency seismic vibrator as a surface acoustic source, wherein the received reflected signals are correlated with a replica of the vibrator frequency pilot signal to produce seismograms of underground structures similar to those obtained with explosives sources.
For the accurate interpretation of seismograms, it is highly desirable to known the velocity of sound waves in the various subterranean layers being mapped. A well-known and commonly used technique for determining the velocity profile of a particular geological region comprises the running of a "velocity log" in an existing borehole in the region. This is typically accomplished with a borehole tool that includes a relatively high-frequency sonic generator displaced a substantial vertical distance from a corresponding sonic signal receiver. Acoustic velocity measurements are obtained by determining the time delay associated with the transmission of the acoustic signal from the transmitter to the receiver.
Another borehole sonic tool in common use is the borehole televiewer. This tool includes a high-frequency sonic transducer that rotates in a plane perpendicular to the direction of the borehole while transmitting and receiving a series of high-frequency sonic pulses. The relative amplitudes of the received sonic pulses are used to characterize the rugosity of the borehole itself.
The above-mentioned techniques are useful for their designed purposes, but they in no way exhaust the possibilities for sonic geophysical exploration and characterization. In particular, the determination of the porosity of subterranean structures is highly desirable as a crucial indicator of potentially exploitable petroleum deposits. There is no currently available borehole sonic technique known to the Applicant for making porosity measurements in the medium surrounding a borehole. It was because of this apparent technological gap that the Applicant proceeded to develop the instant invention.